The present invention relates to emission tomography and in particular to a method and apparatus for applying scatter and attenuation correction to emission tomography images using anatomy inferred from an atlas.
Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are nuclear medicine diagnostic imaging techniques used to measure the three-dimensional distribution of a radiopharmaceutical within the body. Brain SPECT and PET imaging techniques are primarly used to measure regional cerebral blood flow in a patient injected with a radiopharmaceutical to assist in the evaluation of stroke and the diagnosis of dementias such as Alzheimer""s disease.
Although SPECT and PET are useful imaging techniques, their poor quantitative accuracy has been an obstacle in the ability to achieve increased diagnostic reliability. Quantitative accuracy of brain SPECT and PET imaging has however been improved significantly through the application of scatter and attenuation correction to SPECT and PET brain images. To be sufficiently accurate, the application of scatter and attenuation correction to SPECT and PET brain images must be guided by the distribution of density within the head. Unfortunately, the distribution of density within the head cannot be obtained from SPECT and PET brain scans and therefore, separate measurements are required.
Transmission imaging has been used to measure the distribution of density within the head to allow scatter and attenuation correction to be applied to SPECT and PET brain images. Unfortunately, the hardware necessary for making transmission measurements is complex, unreliable and requires extensive maintenance. Also, the need to make transmission imaging measurements in addition to the SPECT or PET brain images, increases the time required to complete the overall imaging procedure. SPECT and PET imaging procedures are themselves lengthy and require a patient to remain motionless to ensure accurate brain images. For sick and elderly patients, this is a difficult task. Adding to the length of the imaging procedure increases the likelihood that patients will not remain motionless. Movement of a patient during the transmission imaging procedure results in inaccurate measurements of the distribution of density within the head. This of course provides an inaccurate guide for the application of scatter and attenuation correction to SPECT and PET brain images. Accordingly, improved methods to increase the diagnostic reliability of emission tomography images are desired.
It is therefore an object of the present invention to provide a novel method and apparatus for applying scatter and attenuation correction to emission tomography images.
Broadly stated, the present invention provides a method and apparatus for applying scatter and attenuation correction to emission tomography images which estimates or xe2x80x9cinfersxe2x80x9d the distribution of density within a region of interest of a patient under observation using a three-dimensional computer model of the region of interest. It has been found that scatter and attenuation correction guided by a computer model of the region of interest under observation produces results similar to those when using transmission images of the actual region of interest as the guide to the application of scatter and attenuation correction.
According to one aspect of the present invention there is provided a method of applying scatter and attenuation correction to emission tomography images of a region of interest of a subject under observation comprising the steps of:
aligning a three-dimensional computer model representing the density distribution within said region of interest with said emission tomography images; and
applying scatter and attenuation correction to said emission tomography images using said aligned computer model as a guide.
In a preferred embodiment, the computer model is in the form of a two-component atlas. During the aligning step, a functional component of the atlas is firstly aligned with the emission tomography images to generate a set of spatial transformation parameters. Following this, an anatomical component of the atlas is aligned with the emission tomography images using the set of spatial transformation parameters.
The atlas may be selected from a database of atlases based on degree of registration with the emission tomography images. Alternatively, multiple atlases maybe combined to yield a resultant atlas which better registers with the emission tomography images.
According to another aspect of the present invention there is provided in an emission tomography imaging system where emission tomography images of a region of interest of a subject are taken for analysis and are corrected for scatter and attenuation, the improvement comprising:
using a three-dimensional computer model approximating the density distribution within the region of interest as a guide to the application of scatter and attenuation correction.
According to still yet another aspect of the present invention there is provided an emission tomography image processing system comprising:
memory storing emission tomography images of a region of interest of a subject;
said memory also storing at least one three-dimensional computer model of said region of interest, said computer model representing the density distribution within said region of interest; and
a processor for registering said computer model with said emission tomography images and for applying scatter and attenuation correction to said emission tomography images using said registered computer model as a guide.
According to still yet another aspect of the present invention there is provided an emission tomography imaging system comprising:
means for taking emission tomography images of a region of interest of a subject to form a three-dimensional image of said region of interest;
memory to store said emission tomography images, said memory also storing at least one three-dimensional computer model of said region of interest, said computer model representing the density distribution within said region of interest; and
a processor for aligning said computer model with said emission tomography images and for applying scatter and attenuation correction to said emission tomography images using said aligned computer model as a guide.
According to still yet another aspect of the present invention there is provided a computer readable medium including computer program code for applying scatter and attenuation correction to emission tomography images of a region of interest of a subject, said computer readable medium including:
computer program code for aligning a three-dimensional computer model representing the density distribution within said region of interest with said emission tomography images; and
computer program code for applying scatter and attenuation correction to said emission tomography images using said aligned computer model as a guide.
The present invention provides advantages in that by using a three-dimensional computer model of the region of interest of a subject under observation that approximates its density as a guide for the application of scatter and attenuation correction to emission tomography images, the need for transmission imaging is obviated. Therefore, in the case of SPECT and PET imaging, the imaging procedures do not need to be lengthened. Also, since the distribution of density in the region of interest under observation is approximated by a three-dimensional computer model, additional hardware is not required to create the guide for the application of scatter and attenuation correction. This makes the present invention significantly less expensive and more flexible than transmission imaging systems. In addition, since a three-dimensional computer model of the region of interest under observation is used as the guide for the application of scatter and attenuation correction, scatter and attenuation correction can be applied retrospectively to existing databases which include significant numbers of emission tomography images for which no transmission imaging measurements were acquired.